Generally in firing guns, the larger and more powerful the ammunition, the larger will be the gun. If the gun is mounted on a vehicle, such as a tank, the mountings must be substantial to transmit the recoil forces to the vehicle and to hold the gun on target for the next shot. In the case of a flexible helicopter frame, however, it is desirable that the gun absorb its own recoil force generated when the gun is fired. In this manner the structural vibration on the helicopter carrier, which is excited by pulsating recoil forces, is lessened and does not interfere with the helicopter flying operation or with the aiming of the gun for subsequent firing.
It has been found that if the peak recoil forces of the gun on its mountings can be reduced substantially, larger weapons may be mounted and used on lighter vehicles. With the reduction of extremely high peaks of the recoil forces, helicopters could carry larger guns and could perform as airborne artillery. Moreover, existing weapons could be fired with greater accuracy.
To this end an M39, 20MM Automatic Gun, normally utilized by fighter and bomber aircraft in forward firing fixed mounts, has been adapted for helicopter use by reducing its peak recoil variation of +2,000 lbs. to -2,000 lbs. to a constant recoil force on the order of approximately 475 lbs. This recoil force is substantially constant when the weapon is fired at 600 rounds per minute. More information concerning this problem and its solution may be had with reference to U.S. Pat. No. 3,677,135 issuing July 18, 1972 to Edward J. Haug, Jr. for Machine Gun Having a Firing System Means for Obtaining Substantially Constant and Minimum Recoil Forces.
The M39 automatic gun is a gas operated, belt-fed and electrically fired revolver type weapon. In its original mode of firings a circuit blade assembly was positioned on the operating slide assembly. When the slide assembly had chambered a cartridge and was in battery position, ready for firing, the circuit blade completed an electrical circuit which caused electrical firing of the cartridge.
The electrical firing system may use alternating or direct current source to fire the weapon. The firing system is composed of the harness assembly which is attached to the drum cradle body, firing circuit blade assembly which is attached to the operating slide, and the firing contact pin assembly which is housed in the drum cradle body. The current flows from the plug connector to the contact blades, continuing toward the firing contact pin only if the operating slide is in battery, thereby positioning the firing circuit blade to complete the circuit.
When the M39 automatic gun has been modified for constant recoil operation, such as taught by the Haug patent earlier referred to, there is an additional switch in the circuit that must be closed before the gun will fire. This additional switch closes when the gun forward momentum (after recoil from firing the previous round) equals half of the ammunition impulse. Because of this additional requirement for constant recoil operation, the firing of the gun by closing the circuit with the operating slide blade assembly (denoting the slide to be in battery position) will not necessarily occur. Only when both conditions are met, that is, the slide is in battery position and the gun forward momentum equals half the ammunition impulse, will the gun fire the next round. More broadly, the slide may be in battery position and waiting for any second event to occur to close a second switch to fire the gun. A rate of fire switch actuator may be another example for dual condition firing for which the present invention is adoptable.
It is with this double criteria before firing, that a slide bounce problem is created, and subsequently solved by the present invention. The slide assembly moves to battery position as it chambers a round in the barrel chamber. Thus, relative to the barrel (and the rest of the gun) the slide moves at a certain rate and with a certain mass. At the same time the gun is firing and recoiling, at a different rate and mass. Hence, the slide doesn't stay in battery position for any appreciable length of time but hits the gun chamber and bounces out again. Under the original operating condition the slide switch would close, the gun would fire, and there would be no problem. However, under the constant recoil mode of operation, the second switch also must close and the gun fired while the slide is in battery position or the gun will not fire. If the second switch closes after the slide switch has opened, due to slide bounce, the firing circuit still is not closed.